Modern radio frequency (RF) communications systems employ complex modulation schemes in which both the magnitude and the angle (phase or frequency) of an RF carrier are modulated to convey information. These complex modulation schemes are used since they increase spectral efficiency. However, they also make it difficult to design an RF transmitter that is energy efficient. Energy efficiency is particularly important in RF transmitters that are battery powered, such as an RF transmitter in a mobile handset, for example. Since the RF transmitter's power amplifier (PA) is usually the component in the RF transmitter that consumes and dissipates the most power, designing the PA so that it operates as efficiently as possible is often one of the principal goals involved in the design of an RF transmitter. Achieving high energy efficiency is met with difficulty, however, particularly in RF transmitters that employ so-called “linear PAs” (e.g., Class A, AB and B PAs). Because the signal envelope of the RF transmitter's RF output varies over time when complex modulation schemes are used, the RF output power must be backed off to avoid signal peak clipping, i.e., to maintain linearity. Unfortunately, the need to back off the output power severely limits the realizable efficiency of the PA and consequently the RF transmitter as a whole, especially when the applied modulation scheme produces an RF output having a high peak-to-average power ratio (PAPR).
One way to circumvent the low energy efficiency limitations that afflict linear PAs is to employ a different type of RF transmitter known as a “polar modulation” transmitter. As illustrated in FIG. 1, a polar modulation transmitter 100 comprises a dynamic power supply (DPS) 102, a phase modulator 104, and a switch-mode PA (SMPA) 106. Operating in the polar domain, the DPS 102 modulates a direct current (DC) power supply voltage VDD(DC) by an input amplitude modulating signal AM(t) to produce a dynamic power supply voltage VDD(t), while the phase modulator 104 modulates an RF carrier by an input phase modulating signal PM(t) to produce a phase modulated RF carrier. The SMPA 106 typically comprises a power field-effect transistor (FET) 108 with its drain configured to receive the DPS voltage VDD(t), via an RF choke 110, and its gate configured to receive the phase-modulated RF carrier RFIN, after first being AC coupled via AC coupling capacitor 112 and biased by a DC bias voltage VBIAS. The DC bias VBIAS and level-to-level swing of RFIN are both set during design so that when the AC-coupled and DC-biased RF input signal RFIN applied to the gate of the power FET 108 switches the SMPA 106 ON and OFF, between compressed and cut-off states. This ‘switch mode’ operation is referred to in the power amplifier arts as “compressed mode” or “C-mode.”
One important property and advantage the SMPA 106 has over linear PAs is that its RF output power POUT is directly proportional to the square of the magnitude of the DPS voltage VDD(t), i.e., POUT ∝VDD2(t). This dependency is exploited in the polar modulation transmitter 100 to superimpose the signal envelope carried by the original input amplitude modulating signal AM(t) (i.e., the “intended” AM) onto the RF output RFOUT. Specifically, as the SMPA 106 translates the phase-modulated RF carrier RFIN to higher RF power it also modulates the RF output RFOUT of the SMPA 106 by the DPS voltage VDD(t) produced by the DPS 102, so that, ideally, the signal envelope of the RF output RFOUT exactly follows the original input amplitude modulating signal AM(t).
Introducing the AM through the drain supply of the SMPA 106 is known in the power amplifier arts as “drain modulation,” and is an ability that avoids having to apply the AM through the RF input of the SMPA 106, i.e., to the gate of its power FET 108. Since the phase-modulated RF carrier RFIN has a constant envelope, the need to back off the RF output power POUT in order to avoid signal peak clipping is therefore obviated. This drain modulation capability together with the fact that the FET 108 is controlled to operate as a switch, rather than a controlled current source as in linear PAs, makes the polar modulation transmitter 100 significantly more energy efficient than RF transmitters constructed from linear PAs.
Although polar modulation transmitters are significantly more energy efficient than transmitters constructed from linear PAs, the bandwidths of the polar domain constant-envelope phase-modulated RF carrier and amplitude modulated DPS voltage VDD(t) are often very wide. For example, as can be seen in FIGS. 2 and 3, which are waveform snippets of the signal envelopes typically observed in communications systems operating according to the Wideband Code Division Multiple Access (W-CDMA) air interface (FIG. 2) and Long-Term Evolution (LTE) interface (FIG. 3), the signal envelopes tend to fall to zero (or near zero) and inflect back from zero (or near zero) abruptly. These sharply-inflecting low-magnitude events occupy a wide bandwidth when represented in the frequency domain, and conventional polar modulation transmitters, like the polar modulation transmitter 100 described above, are unfortunately unable to resolve them at their RF outputs RFOUT. The invention described below addresses and provides a solution to this problem.